This invention relates generally to an electrophotographic printing machine, and more particularly concerns an electrophotographic printing machine adapted to reproduce either an opaque original document or a transparency.
In the process of electrophotographic printing, a charged photoconductive member is exposed to a light image of an original document. The irradiated areas of the photoconductive surface are discharged selectively recording thereon an electrostatic latent image of the original document. A development system moves a developer mix of carrier granules and toner particles into contact with the latent image recorded on the photoconductive surface. The toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image thereon. Thereafter, the toner powder image is transferred from the latent image to a sheet of support material. After the toner powder image has been transferred to the sheet of support material, the sheet of support material passes through a fixing device which causes the toner powder image to permanently adhere thereto. The foregoing briefly describes the general concept of electrophotographic printing.
Multi-color electrophotographic printing is substantially the same as has heretofore been described. However, a plurality of single color light images are employed to record single color electrostatic latent images on the photoconductive surface. Each single color electrostatic latent image is developed with the appropriately colored toner particles. Successive single color toner powder images are transferred to the sheet of support material forming a multi-layered toner powder image. The multi-layered toner powder image is permanently affixed to the sheet of support material resulting in a multi-color copy of the original document being reproduced.
A wide variety of machines have been developed for reproducing specific types of original documents. For example, machines are presently in wide commercial use for reproducing microfilm. Exemplary patents describing microfilm reproduction machines are U.S. Pat. No. 3,424,525 issued to Towers et al. in 1969; U.S. Pat. No. 3,542,468 issued to Blow, Jr. in 1970; and U.S. Pat. No. 3,547,533 issued to Stokes et al. in 1970. In machines of this type, an enlarged copy of a microfilm input is reproduced. However, it is extremely difficult to produce good pictorial quality copies. Moreover, until recently, color transparencies were not readily reproducable. However, with the advent of multi-color electrophotographic printing, the capability to copy a color slide was developed. A suitable electrophotographic printing machine designed to copy slides is described in co-pending application, Ser. No. 540,617 filed in 1975. As described therein, the light image of the color transparency passes through a screen positioned on the machine platen. The modulated light image is filtered as it passes through the machine optics to selectively discharge the charged portion of the photoconductive surface. Successive single color electrostatic latent images are formed and developed. These powder images are transferred to the sheet of support material, in superimposed registration with one another, resulting in a copy corresponding to the light image of the color transparency being projected through the machine optics. A field lens and screen are positioned on the platen. The field lens converges the diverging light rays of the light image while the screen modulates the light image. However, it has been found that the screen and field lens are frequently unwieldy to handle when converting from an opaque copying machine to a transparency copying machine. Moreover, the possibility exists that the screen and field lens may be positioned erroneously on the platen or mislayed and lost when removed therefrom.
Accordingly, it is the primary object of the present invention to improve the structure holding the screen and field lens adjacent to the printing machine platen so as to facilitate the removal thereof therefrom when switching modes of operation.